Fertilizing Rhododendrons: How And When Do You Fertilize Rhododendrons

Fertilizing A Potted Rhododendron Plant
(Image credit: ronstik)

Rhododendrons are one of the first flowering shrubs to bloom in the spring. The popular bushes can be long-lived and healthy if given proper care. To get the most flowering power though, you may need to fertilize. 

So, when do you fertilize rhododendrons? Read on for tips about the best fertilizer for rhododendrons and when/how to get the job done.

Best Fertilizer for Rhododendrons

If you have planted your shrubs in fertile soil, fertilizing rhododendrons is not a necessity. However, those rhododendrons growing in poor soil need feeding. Likewise, plants grown with mulch that depletes nitrogen in the soil, such as fresh wood chips, will probably do better with a regular rhododendron fertilizer schedule.

If your shrubs require feeding, you not only need to know when and how to fertilize rhododendron bushes, but you’ll want to use the best fertilizer for rhododendrons. They require a complete food dressing at appropriate intervals.

Although you’ll find some products labeled as specific for rhododendrons and azaleas, these are not necessarily the best for your plants since they acidify the soil. You only need this if your soil is not sufficiently acidic for the plants. Plants generally require three nutrients to thrive: nitrogen, phosphorus, and potassium

Most fertilizers contain a mix of these three nutrients, with the percentages of each listed in that order on the label. A complete fertilizer contains all three of these in different percentage amounts.

Rhododendron Fertilizer Schedule

You’ll want to use a particular fertilizer composition for the different stages of your rhododendron growth. At planting time, use 10-10-6 fertilizer before you water the plant in. In early spring, the rhododendron buds swell. At this time, apply a complete 10-8-6 fertilizer. Apply another light dose of this fertilizer when the leaves emerge. In autumn, applying organic compost or manure is sufficient.

How to Fertilize Rhododendron Bushes

You can choose between various types of fertilizer for your plants. Granular fertilizer comes in grains that are sprinkled on the soil and watered in. Liquid or water-soluble fertilizer is mixed with water and then poured into the soil. 

If you are concerned about cost, granular fertilizer is the best fertilizer for rhododendrons since it is the cheapest. Liquid fertilizer passes quickly through the soil and application must be repeated more frequently. A general rule of thumb is to use one level tablespoon (15 mL.) of fertilizer for every foot (30.5 cm.) of plant growth. 

You can purchase granular fertilizer that is labeled slow-release. It releases its nutrients slowly, over time.

Teo Spengler

Teo Spengler has been gardening for 30 years. She is a docent at the San Francisco Botanical Garden. Her passion is trees, 250 of which she has planted on her land in France.